1. Field of the Invention
The present invention relates to a method and apparatus for monitoring or detecting sub-micron particles on a novel principle which is completely different from the conventional techniques.
The method according to the present invention is advantageously applicable to monitor and control impurity particles in fluids such as pure water and ultrapure water used in electronics industries, biotechnology, medical and pharmaceutical application and foods industries. The present method can be used to evaluate the performance of separation membranes and filtration systems.
2. Related Arts
The conventional methods for monitoring or detecting particles in fluid are classified into following four categories:
(1) Shadow system in which decrement of light intensity caused by travelling particles in fluid passing across an optical axis of parallel ray. PA1 (2) Microscope system in which fine particles in fluid are caught by a membrane filter or the like and are observed or counted by a electron scanning microscope. PA1 (3) Light scattering system in which fluid is irradiated with an intensive light such as a laser beam and the resulting scattered light is collected by a lens so that the focused light is detected by an photo-multiplier PA1 (4) Imaging system in which a fluid is irradiated with a light and the resulting contrast of light is detected by a photo-diode alley and an image of particles in the fluid is formed by a computer.
New techniques such as ultrasonic scattering technique are also proposed.
In the case of the shadow system (1), however, detection of fine particles is limited to the particle size of about 1 .mu.m and hence this detection system can not be used for sub-micron particles. In the microscope system (2), more than half day is required to obtain the result.
The light scattering system (3) is the main current of development in particle counters or detectors and now ultra-fine particles having the particle size of less than 0.07 .mu.m can be detected by using a light source having shorter wave length such as argon laser. In fact, Japanese patent laid-open No. 4-39,635 discloses a technique to determine the precise number of fine contaminant particles each having the particle size of lower than 0.07 .mu.m contained in ultrapure water. This patent proposes to use two detectors each receive the scattered light so that a particle counter produces a signal when two detectors detect the scattered light simultaneously. This system, however, requires a high-power laser as well as very sensitive photo-multiplier, resulting in a large costly system. Still more, in this system, precise alignment between an axis of fluid stream containing particles to be detected and an optical axis is required in order to assure the reliability of measurement. Japanese patent laid-open No. 62-803 discloses an automated apparatus which facilitates this alignment.
Japanese patent laid-open No. 63-19535 discloses a variation of the imaging system (4). In this patent, a laser beam impinges vertically to a flow of sample liquid and the diffracted and scattered light is passed through a Fourier-transformation optical system or a lens to produce a Fraunhofer diffraction image which is treated in order to evaluate fine particles in the liquid. In this patent, a diameter of a laser beam is enlarged to obtain a parallel ray which is directed to the sample liquid. This system requires a complicated computer system.
Therefore, an object of the present invention is to provide a method which permits to detect fine particles of sub-micron as contaminant in fluid, in particular, pure water or ultrapure water by a simple and very economical apparatus.